1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) system for obtaining configuration data such as a density distribution image of a specific atomic nucleus of an object (e.g., a living subject) or function data such as a spectroscopy by utilizing a magnetic resonance (MR) phenomenon and, more particularly, to an MRI system for obtaining an image of a brain surface structure.
2. Description of the Related Art
In an MR phenomenon, a specific atomic nucleus having a non-zero spin and a magnetic moment based on the spin resonantly absorbs only an electromagnetic wave of a specific frequency in a static field. The atomic nucleus resonates at an angular frequency .omega.0 (.omega.0=2.pi..nu.0 .nu.0: Larmor frequency) given by the following equation: EQU .omega.0=.gamma.H0
where .gamma. is the gyromagnetic ratio unique to a type of atomic nucleus, and H0 is the static field strength.
In an MRI system for obtaining data for living subject diagnosis by utilizing the above phenomenon, a magnetic resonance (MR) signal as an electromagnetic wave having the same specific frequency as described above excited after resonance absorption is detected and is subjected to signal processing, thereby obtaining diagnosis data reflecting MR data such as an atomic nuclear density, a longitudinal relaxation time T1, a transverse relaxation time T2, a blood flow or flow of cerebrospinal fluid (CSF), or a chemical shift, e.g., a slice image of a selected slice of an object without invasion.
when diagnosis data is acquired by utilizing the MR phenomenon, MR signals induced by exciting every portion of an object placed in a static field can be acquired, in principle. In an actual system, however, due to limitations on a system configuration and clinical requirements for a diagnostic image, an MR phenomenon is excited in a specific, limited portion and MR signals therefrom are acquired.
In this case, the specific portion serving as an imaging object is a slice portion having a given thickness. MR signal data such as echo signals or free induction decay (FID) signals from this slice portion are acquired by executing a large number of MR excitation/data acquisition sequences, and these acquired data are subjected to image reconstruction processing by, e.g., a two-dimensional Fourier transformation method, thereby generating an MR image of the specific slice portion.
A clinical application using an MRI system will be described below. More specifically, in order to perform a surgical treatment, i.e., operation of an intracranial disease, a brain surface structure including cerebral grooves, serves as an important criterion upon detection of the position of a lesion locally present in or under a cortex. Therefore, it is desired to obtain an accurate position of the brain surface structure before an operation. In order to obtain the accurate position of the brain surface structure, some attempts are made to obtain a brain surface structure image representing the brain surface structure as well as the cerebral grooves. Such attempts will be explained below.
One of these attempts is a method of performing normal proton imaging using an appropriate head coil. In this method, a cylindrical coil which surrounds a head of a patient can be used as the head coil. When the head coil operates over the entire head, signals from the entire head are acquired. As a result, an image is obtained in which data of a deep portion under the brain surface are unnecessarily superposed. Therefore, it is not easy to discriminate the brain surface structure from such an image, and, hence, the above-mentioned diagnostic requirements cannot be satisfied.
Another attempt is a method of performing normal proton imaging using a surface coil. The surface coil exhibits a high sensitivity for only a portion adjacent to the coil. When this surface coil is used, only MR signals near a body surface can be acquired. Therefore, this method is often used in MRI when a portion of interest is present near the body surface. With this method, however, only signals from a subcutaneous fat of a surface layer portion are acquired according to the sensitivity characteristics of the surface coil, and the above-mentioned diagnostic requirements cannot be satisfied, either.
With the above-mentioned two methods, an image which can properly express a brain surface structure cannot be obtained.
In the conventional methods, MR data from, e.g. the CSF and MR data from a fatty area similarly acquired without being distinguished from each other. Therefore, it is difficult to obtain an effective brain surface structure image for diagnosing the position of a lesion locally present in or under a cortex.